Study On Structure Optimization And Performance Of BaCo_0.7Fe_0.3O_3-δ Based Cathode Materials

Solid oxide fuel cells(SOFCs)are potential energy devices,which can directly convert chemical energy into electrical energy.They have attracted many people’s attention because of their environmental benignity,fuel flexibility and high energy conversion efficiency.However,the high operating temperature(800-1000℃)of fuel cells will cause several problems related to material durability and cost.Hence,reducing the operating temperature to medium and low temperature range(500-700℃)is very important for commercializing application of fuel cells.As the working temperature decreases,the output performance of fuel cells decreases significantly and the performance of the fuel cells deteriorates in toxic environment(CO2,H2O),which limit the development of fuel cells.In order to solve these problems,people are committed to developing a cathode with high activity in the medium and low temperature range.In recent years,BaCo0.7Fe0.3O3-δ(BCF)has attracted more and more attention mainly of its sufficient oxygen surface activation and diffusion activity,excellent mixed ion and electron ion conductivity(MIEC).However,the hexagonal phase of BCF at room temperature,poor stability and slow oxygen diffusion limit its wide application.In this paper,we mainly study the improvement of electrocatalytic activity and stability of cathode materials with optimized structure and dopped elements at medium and low temperature.Firstly,the ORR activity and stability of BaCo0.7Fe0.3O3-δ cathode material were improved by double doping.BaCo0.7Fe0.3O3-δ perovskite material transformed from the original hexagonal structure to a cubic phase structure with high stability,which was contributed by La/Nb double doping.In addition,Ba ion at A site was replaced by La ion with smaller radius,which increased the oxygen vacancy concentration and thus improved ORR activity of BCF cathode.At 700℃air,the polarization impedance of the Ba0.9La0.1Co0.3Fe0.2Nb0.1O3-δ cathode was 0.073 Ω·cm2,which was only 34.9%of that of BCF.In addition,the electrochemical impedance spectroscopy(EIS)test showed that after exposure to air containing 10%CO2 for 2h,the polarization impedance increased by only 16.4%compared with the initial one.The doping of high-valence metals at B site of BaCo0.7Fe0.3O3-δ perovskite may increase the acidity on the surface,leading to enhanced resistance to CO2.A peak power density of 812 mW·cm-2 was achieved with the BLCFN cathode at 650℃.Secondly,the fibrous composite cathode was synthesized by using a coaxial electrospinning technique,which remarkably improved the active site of the cathode and CO2 durability.Ba0.9La0.1Co0.3Fe0.2Nb0.1O3-δ(BLCFN)and Sm0.2Ce0.8O2-δ(SDC)nanoparticles uniformly adhered to each other in the fiber tube,which not only prolonged the length of the three-phase reaction interface(air,cathode and electrolyte)but also created more oxygen active site.This cathode achieved a polarization resistance of 0.05 Ω·cm2 at 700℃,which was lower than that of single-phase BLCFN fiber cathode(0.069 Ω cm2)and BLCFN powder cathode(0.073Ω· cm2).In addition,the maximum power density of BLCFN-SDC fuel cell can reach 865 mW·cm-2.Additionally,adding a certain amount of SDC also improved the CO2 tolerance of the cathode.In high concentration CO2 atmosphere(10 vol%CO2),the Rp of fibrous BLCFNSDC increased only 16%,which was much lower than that of single-phase BLCFN with two different morphologies(43%,46%).The high CO2 durability was likely attributable to the low CO2 adsorption and reactivity of SDC.Finally,the use of alkaline cation K+doped BCFY can improve the proton migration ability and ORR activity,and promote surface to form BaCoO3 nanoparticles with higher catalytic activity in the case of water vapor,that exhibited remarkable electrocatalytic activity for oxygen reduction reaction(ORR)while maintaining excellent tolerance to water vapor.Electrochemical tests showed that the impedance of BKCFY is 0.063 Ω·cm2 at 700℃ in air,which was significantly lower than that of BCFY(700℃,0.074 Ω·cm2).The impedance of BKCFY was 0.048 Ω·cm2 when a certain amount of water vapor was introduced at 700℃,which was 24.8%lower than that without water vapor.What’s more,ECR and TG tests showed that K+doping led to more oxygen vacancies,which was conducive to the increase of proton concentration.In addition,when a stable polarization current density of 700 mA cm-2 was applied to the single cell,it maintained stable performance at 700℃.